2015-2016 winter outlook

2015-2016 Winter Outlook

Overview

Extended Video Discussion

It looks like the Mid-Atlantic region is going to experience another snowy winter with numerous coastal storms – and there can even be a blockbuster snowstorm or two. Temperatures are likely to average out to near normal or slightly above-normal for the winter season in the Mid-Atlantic region, but there will be occasional Arctic air outbreaks as well. Warmer-than-normal weather is likely across much of the northern US and colder-than-normal conditions are expected in much of the Deep South. Last winter, the Mid-Atlantic region suffered through a snowy and cold winter and farther up the coastline, Boston, Massachusetts experienced its snowiest winter ever. If this winter does indeed produce more snow than normal in DC, Philly, and New York then it would be the third in a row which is quite uncommon around here.

Key factors

There are several key factors listed below that were involved with this year’s winter outlook:

Strong El Nino conditions focused in the central part of the tropical Pacific Ocean

El Nino

A very strong El Nino event is well underway in the tropical Pacific Ocean and it will play a vital role in our upcoming winter weather. Very strong El Nino events were taking place on average about once a decade in the latter part of last century (1972-1973, 1982-1983, 1997-1998); however, the first decade of this century did not feature a comparable very strong El Nino. Instead, there were three moderate El Nino events to begin this century in the following years: 2002-2003, 2006-2007 and 2009-2010. The current El Nino is more analogous in strength to those episodes which occurred in the 1970’s, 1980’s and 1990’s.

The current El Nino began during the latter part of last year and strengthened considerably earlier this year. An El Nino of this magnitude is likely to increase the intensity of the southern branch of the jet stream during the winter season and it'll act to pump a lot of moisture into the upper-level wind flow. The southern branch of the jet stream is crucial for the development of storms in the southwestern US (e.g., California) that can then travel across the southern US and ride up the east coast (i.e., coastal storms). Strong El Nino’s have produced some blockbuster snowstorms for the Mid-Atlantic region in the past (e.g., February 1983) and the combination of the “pumped up” southern branch of the jet stream and warmer-than-normal sea surface temperatures near the US east coast raises the prospects for just such an event or two this winter.

While the magnitude of El Nino is very important as far as its potential impact on our winter weather, so is its location. A “centrally-based” El Nino is one in which the greatest sea surface temperature anomalies are located in the central tropical Pacific Ocean whereas an “eastern-based” El Nino has the warmest water relative–to-normal much closer to the west coast of South America. A “centrally-based” El Nino is more likely to be associated with an upper-level ridge of high pressure along the west coast of North America and this, in turn, can help to push cold air masses into the northeastern US. On the other hand, an “eastern-based” El Nino (e.g., 1997-1998) with its warming focused near the west coast of South America would more likely result in southwesterly winds across the eastern US producing warmer-than-normal wintertime conditions. I believe the most recent changes in the overall sea surface temperature pattern across the tropical Pacific Ocean favor the idea of a “centrally-based” El Nino this winter and multiple computer forecast models tend to support this notion (NOAA CFSv2, JAMSTEC, UKMET).

Northeastern Pacific Ocean warm pool of water

El Nino is not the only game in town as far as the winter outlook is concerned and it is important to look elsewhere. Another important region that may impact our winter weather is the northeastern Pacific Ocean where a warm pool of water has persisted for the past couple of winters. It appears that this pattern will continue through another winter season and this should help to generate high pressure ridging along the west coast of North America which, in turn, should push cold air outbreaks into the northeastern US.

Two independently-made sea surface temperature anomaly forecasts made for the upcoming winter season (below) tend to agree on the major players across the all-important Pacific Ocean. NOAA’s CFSv2 suggests the El Nino conditions will become more “centrally-based” in the Pacific Ocean during the winter season and that there will be a continuation of warmer-than-normal water in the northeastern Pacific Ocean. The JAMSTEC forecast model is produced by Japan’s Meteorological Agency and it tends to agree with the NOAA/CFSv2 forecast in the overall sea surface temperature pattern in the Pacific Ocean.

Analog years

While sea surface temperatures never repeat exactly from one year to another, it is still quite useful for long-range forecasting purposes to find “analog years” in which there are somewhat similar oceanic patterns to current conditions. Indeed, I believe the following winter seasons featured somewhat analogous sea surface temperature patterns compared to today’s environment and they can provide us a clue as to what type of winter we may experience around here: 1997-1998, 1991-1992, 1982-1983, 1957-1958, 2009-2010 and 1986-1987. The one overriding characteristic of these analog years is that they all featured moderate or strong El Nino events in the tropical Pacific Ocean. Note- even though there was a strong El Nino event in 1972-1973, it is not included in my analog year list as there were significant sea surface temperature differences in the northeastern Pacific Ocean compared to the today.

These particular analog winter seasons resulted in overall temperatures that averaged above-normal across the northern US and below-normal in southern areas of the US. The Mid-Atlantic region was sandwiched between these two zones with near normal or slightly above normal conditions on average during these analog years.

Average Dec to Feb temperature anomaly pattern for the analog years; courtesy NOAA

An interesting finding with respect to temperatures in these chosen analog years is revealed when looking at “month-to-month” anomalies. There was a clear tendency for the eastern US to become progressively colder relative-to-normal as the winter season evolved. Warmer-than-normal temperatures (seen in yellow, orange, red) were quite expansive in December across the eastern US during these analog years and then they retreated somewhat in January and then even more so during February.

As far as precipitation is concerned, these analog years featured on average wetter-than-normal conditions in the southern states as well as along much of both coastlines. As with temperatures, this is a somewhat typical pattern seen during winter seasons in El Nino years and is reflective of the “pumped up” southern branch of the jet stream.

Average Dec to Feb precipitation anomaly pattern for the analog years; courtesy NOAA

“High-latitude blocking”

If the current strong El Nino in the tropical Pacific Ocean was the only player on the field this winter then much of the US would likely experience a warmer-than-normal winter as El Nino is one of nature's best ways to redistribute heat around the world. There is an atmospheric phenomenon, however, that can counteract El Nino's potential warming in the Mid-Atlantic region during the winter season and it is known as "high-latitude blocking". As a result, it is quite important in the long-range forecasting of winter season temperatures in the Mid-Atlantic region to evaluate the prospects for “high latitude blocking”.

“High-latitude blocking” during the winter season is generally characterized by persistent high pressure in northern latitude areas such as Greenland and northern Canada. Without this type of pattern in the atmosphere, it would be quite difficult to get sustained cold air masses in the Mid-Atlantic region during the winter season; especially, during El Nino (warm) events. Coastal storms in the I-95 corridor absent sustained cold air would be much more likely to generate rain or snow changing to rain in the big cities along I-95.

The Arctic Oscillation signal and “high-latitude blocking”

“High-latitude blocking” is tracked by meteorologists through indices known as the Arctic Oscillation (AO) and its closely-related cousin called the North Atlantic Oscillation (NAO). The Arctic Oscillation refers to opposing atmospheric pressure patterns in middle and high latitudes. When the AO is positive, for example, surface pressure is low in the polar region and this helps the mid-latitude jet stream to blow strongly and consistently from west-to-east keeping Arctic air locked up in the polar region. When the AO index is negative, there tends to be high pressure in the polar regions (i.e., “high-latitude blocking”), weaker zonal winds, and greater movement of polar air into the middle latitudes. While the AO and NAO indices are primarily used during by forecasters during the winter season, trends in summer and fall seasons can provide important clues about the ensuing winter season.

Evidence shows that when AO reaches negative values on a consistent basis during the month of July, the subsequent winter season typically will have frequent “negative” AO periods which are correlated with “high-latitude blocking” patterns. As it turned out, the AO signal this past July averaged out to be one of the most negative in the past 50 years and every single one of the ten most negative AO index readings during the month of July featured negative AO index values during subsequent winters which would typically give the northeastern US plenty of cold air to work with. In addition, AO index trends in autumn seasons also have been shown to be useful predictors for subsequent winter seasons. Specifically, negative AO index values in October typically translate to negative values during the following winter season. In summary, the low mean AO value of this past summer negatively biases the upcoming winter AO and if the current negative AO persists for much of the fall, this too will likely increase the odds of negative AO during the winter season.

Arctic Oscillation index (actual black; forecast red); courtesy NOAA

The snowpack signal and “high-latitude blocking”

In addition to the Arctic Oscillation signal, snowpack in the northern hemisphere during the autumn season has also been found to be an important predictive factor with respect to “high-latitude blocking” patterns during subsequent winter seasons. In fact, research studies have actually pinpointed the region in Siberia below 60°N during the month of October as critical with respect to the likelihood of “high-latitude blocking” atmospheric patterns during the following winter season. If snowpack is above-normal and consistently expanding during October in that particular part of Siberia, research studies suggest there is an increased chance for more frequent “high-latitude blocking” configurations in subsequent winter months.

Indeed, the snow anomaly chart (below) shows above-normal snowfall at the end of September for the northern hemisphere although not quite at the extreme levels of a year ago. In addition, there has been a significant increase in snowpack across Siberia during the first half of October including in the region south of 60°N (white area in maps below) and I expect this trend to continue during the second half of the month. For Eurasia as a whole, the snowpack at the end of September was the 18th highest in the last 47 years and it was the 14th highest across the entire northern hemisphere (source Rutgers Snow Lab).

Low solar activity and “high-latitude blocking”

Research has shown that low solar activity tends to be correlated with frequent “high-latitude blocking” patterns and we are now experiencing one of the weakest solar cycles (#24) in more than a century. The “analog years” plot below shows surface-level height anomalies in low solar activity years (solar minimum phases) and high pressure dominates near Greenland and Iceland (orange, red). In addition to solar cycle 24 being a historically weak one, we have likely exited its solar maximum phase (indicated by arrow) - usually the most active time in a given solar cycle - and are now headed towards the next solar minimum. As a result, odds favor low solar activity during this upcoming winter season which has been found to be well correlated with frequent “high-latitude blocking” scenarios.

The bottom-line

Given the expected oceanic surface temperature patterns around the world for this winter season and the likelihood for “high-latitude blocking” events, I believe the Mid-Atlantic region from DC-to-Philly-to-New York City will experience normal to slightly above normal temperatures (0.0°C to +1.0°C); however, there will be occasional Arctic air outbreaks. As the winter season progresses, the weather should turn increasingly colder “relative-to-normal” in the Mid-Atlantic region following a mild November and likely a warmer-than-normal December.

The overall weather pattern should be quite stormy with numerous coastal storms and above-normal snowfall in DC, Philly and New York City with the heaviest snow likely falling during the second half of the winter season. In addition, there is the chance for a blockbuster snow event or two given the expected “El Nino-enhanced” coastal storms and, if this were to occur, it would most likely happen during the latter half of winter. Look for at least 20-30 inches in the DC metro region during the upcoming winter season, 30-40 inches in Philly, and 35-45 inches in the NYC metro region.

Elsewhere, much of the northern US should feature above-normal temperatures while much of the southern half of the nation will be colder-than-normal. This would be a dramatic difference from recent winters in the Northern Plains and Upper Midwest where they have suffered through bitter cold weather conditions. Precipitation should be above-normal in California - alleviating their drought significantly - and higher-than-normal across the southern US and up along the eastern seaboard.